It is generally conceded that marijuana is widely used in the United States today, particularly among the young, and that the use of this drug is generally on the increase. In North America, marijuana, like tobacco, is consumed primarily by smoking. Despite this, essentially nothing is known about the biological effects of marijuana on the lung. These studies will objectively, quantitatively, and precisely define the biochemical, metabolic, enzymatic, ultrastructural, and functional effects of marijuana on the lung in vivo and on the pulmonary alveolar macrophage in vitro. If marijuana is a potential health hazard, these studies will clarify that issue. Objective data on marijuana of this nature are crucial to defining policies on its use.